The importance of the Thr<SUP>17</SUP> residue of phospholamban as a phosphorylation site under physiological and pathological conditions

Abstract

The sarcoplasmic reticulum (SR) Ca<SUP>2+</SUP>-ATPase (SERCA2a) is under the control of an SR protein named phospholamban (PLN). Dephosphorylated PLN inhibits SERCA2a, whereas phosphorylation of PLN at either the Ser<SUP>16</SUP> site by PKA or the Thr<SUP>17</SUP> site by CaMKII reverses this inhibition, thus increasing SERCA2a activity and the rate of Ca<SUP>2+</SUP> uptake by the SR. This leads to an increase in the velocity of relaxation, SR Ca<SUP>2+</SUP> load and myocardial contractility. In the intact heart, β-adrenoceptor stimulation results in phosphorylation of PLN at both Ser<SUP>16</SUP> and Thr<SUP>17</SUP> residues. Phosphorylation of the Thr<SUP>17</SUP> residue requires both stimulation of the CaMKII signaling pathways and inhibition of PP1, the major phosphatase that dephosphorylates PLN. These two prerequisites appear to be fulfilled by β-adrenoceptor stimulation, which as a result of PKA activation, triggers the activation of CaMKII by increasing intracellular Ca<SUP>2+</SUP>, and inhibits PP1. Several pathological situations such as ischemia-reperfusion injury or hypercapnic acidosis provide the required conditions for the phosphorylation of the Thr<SUP>17</SUP> residue of PLN, independently of the increase in PKA activity, i.e., increased intracellular Ca<SUP>2+</SUP> and acidosis-induced phosphatase inhibition. Our results indicated that PLN was phosphorylated at Thr<SUP>17</SUP> at the onset of reflow and immediately after hypercapnia was established, and that this phosphorylation contributes to the mechanical recovery after both the ischemic and acidic insults. Studies on transgenic mice with Thr<SUP>17</SUP> mutated to Ala (PLN-T17A) are consistent with these results. Thus, phosphorylation of the Thr<SUP>17</SUP> residue of PLN probably participates in a protective mechanism that favors Ca<SUP>2+</SUP> handling and limits intracellular Ca<SUP>2+</SUP> overload in pathological situations.